Optical filtering with axial whispering gallery modes on the surface of tapered optical fibers

We study the Q-factors of whispering gallery modes on the surface of standard optical fibers using both spectral and ring-down measurements and show that annealing by the fire torch yields Q-factors higher than 10⁷. In this case, however, variations of the effective radius are introduced, that make the mode spectrum complicated as a number of modes with a nonzero axial component of the wave vector appear. We demonstrate that tapering the fiber sample allows to evade this complexity and to build up an efficient filter operating in transmission.